/*
 * Copyright 2012 Hongdian, Inc. All rights reserved.
 */
package com.sn.utils.crypto;

/**
 * <a href="http://en.wikipedia.org/wiki/Hexadecimal">Hexadecimal</a> encoder
 * and decoder.
 * <p/>
 * This class was borrowed from Apache Commons Codec SVN repository (rev.
 * {@code 560660}) with modifications to enable Hex conversion without a full
 * dependency on Commons Codec. We didn't want to reinvent the wheel of great
 * work they've done, but also didn't want to force every Shiro user to depend
 * on the commons-codec.jar
 * <p/>
 * As per the Apache 2.0 license, the original copyright notice and all author
 * and copyright information have remained in tact.
 * 
 * @see <a href="http://en.wikipedia.org/wiki/Hexadecimal">Wikipedia:
 *      Hexadecimal</a>
 * @since 0.9
 */
public class Hex {

	/**
	 * Used to build output as Hex
	 */
	private static final char[] DIGITS = { '0', '1', '2', '3', '4', '5', '6',
			'7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f' };

	/**
	 * Encodes the specifed byte array to a character array and then returns
	 * that character array as a String.
	 * 
	 * @param bytes
	 *            the byte array to Hex-encode.
	 * @return A String representation of the resultant hex-encoded char array.
	 */
	public static String encodeToString(byte[] bytes) {
		char[] encodedChars = encode(bytes);
		return new String(encodedChars);
	}

	/**
	 * Converts an array of bytes into an array of characters representing the
	 * hexidecimal values of each byte in order. The returned array will be
	 * double the length of the passed array, as it takes two characters to
	 * represent any given byte.
	 * 
	 * @param data
	 *            byte[] to convert to Hex characters
	 * @return A char[] containing hexidecimal characters
	 */
	public static char[] encode(byte[] data) {

		int l = data.length;

		char[] out = new char[l << 1];

		// two characters form the hex value.
		for (int i = 0, j = 0; i < l; i++) {
			out[j++] = DIGITS[(0xF0 & data[i]) >>> 4];
			out[j++] = DIGITS[0x0F & data[i]];
		}

		return out;
	}

	/**
	 * Converts an array of character bytes representing hexidecimal values into
	 * an array of bytes of those same values. The returned array will be half
	 * the length of the passed array, as it takes two characters to represent
	 * any given byte. An exception is thrown if the passed char array has an
	 * odd number of elements.
	 * 
	 * @param array
	 *            An array of character bytes containing hexidecimal digits
	 * @return A byte array containing binary data decoded from the supplied
	 *         byte array (representing characters).
	 * @throws IllegalArgumentException
	 *             Thrown if an odd number of characters is supplied to this
	 *             function
	 * @see #decode(char[])
	 */
	public static byte[] decode(byte[] array) throws IllegalArgumentException {
		String s = new String(array);
		return decode(s);
	}

	/**
	 * Converts the specified Hex-encoded String into a raw byte array. This is
	 * a convenience method that merely delegates to {@link #decode(char[])}
	 * using the argument's hex.toCharArray() value.
	 * 
	 * @param hex
	 *            a Hex-encoded String.
	 * @return A byte array containing binary data decoded from the supplied
	 *         String's char array.
	 */
	public static byte[] decode(String hex) {
		return decode(hex.toCharArray());
	}

	/**
	 * Converts an array of characters representing hexidecimal values into an
	 * array of bytes of those same values. The returned array will be half the
	 * length of the passed array, as it takes two characters to represent any
	 * given byte. An exception is thrown if the passed char array has an odd
	 * number of elements.
	 * 
	 * @param data
	 *            An array of characters containing hexidecimal digits
	 * @return A byte array containing binary data decoded from the supplied
	 *         char array.
	 * @throws IllegalArgumentException
	 *             if an odd number or illegal of characters is supplied
	 */
	public static byte[] decode(char[] data) throws IllegalArgumentException {

		int len = data.length;

		if ((len & 0x01) != 0) {
			throw new IllegalArgumentException("Odd number of characters.");
		}

		byte[] out = new byte[len >> 1];

		// two characters form the hex value.
		for (int i = 0, j = 0; j < len; i++) {
			int f = toDigit(data[j], j) << 4;
			j++;
			f = f | toDigit(data[j], j);
			j++;
			out[i] = (byte) (f & 0xFF);
		}

		return out;
	}

	/**
	 * Converts a hexadecimal character to an integer.
	 * 
	 * @param ch
	 *            A character to convert to an integer digit
	 * @param index
	 *            The index of the character in the source
	 * @return An integer
	 * @throws IllegalArgumentException
	 *             if ch is an illegal hex character
	 */
	protected static int toDigit(char ch, int index)
			throws IllegalArgumentException {
		int digit = Character.digit(ch, 16);
		if (digit == -1) {
			throw new IllegalArgumentException("Illegal hexadecimal charcter "
					+ ch + " at index " + index);
		}
		return digit;
	}

}
